No species lasts forever — extinction is part of the

evolution of life.

But at least five times, a biological catastrophe has engulfed

the planet, killing off the vast majority of species from water
and land over a relatively short geological interval.

The most famous of these mass extinction events — when

an asteroid slammed into Earth 66 million years ago ,
dooming the dinosaurs and many other species — is also the
most recent. But scientists say it won’t be the last.

Many researchers argue we’re in the middle of a sixth mass

extinction, caused not by a city-size space rock but by the
overgrowth and transformative behavior of a single
species — Homo sapiens. Humans have destroyed habitats
and unleashed a climate crisis .

Calculations in a September study published in the journal

PNAS have suggested that groups of related animal species
are disappearing at a rate 35% times higher than the
normally expected rate.

And while every mass extinction has winners and losers,

there is no reason to assume that human beings in this case
would be among the survivors.

In fact, study coauthor Gerardo Ceballos thinks the opposite

could come to pass, with the sixth mass extinction
transforming the whole biosphere, or the area of the world
hospitable to life — possibly into a state in which it may be
impossible for humanity to persist unless dramatic action is
taken.

“Biodiversity will recover but the winners (are) very difficult

to predict. Many of the losers in these past mass extinctions
were incredibly successful groups,” said Ceballos, a senior
researcher at the Institute of Ecology at the National
Autonomous University of Mexico.

While the causes of the “big five” mass extinctions varied,

understanding what happened during these dramatic
chapters in Earth’s history — and what emerged in the
aftermath of these cataclysms — can be instructive.

“Nobody’s seen these events but they’re on a scale that

might be repeated. We’ve got … (to) learn from the past
because that’s our only data set,” said Michael Benton, a
professor of vertebrate paleontology at Bristol University in
the United Kingdom who is the author of the new book
“Extinctions: How Life Survives, Adapts and Evolves .”

A really bad day: Dino-killing asteroid and the iridium anomaly

While paleontologists have studied fossils for centuries, the
science of mass extinction is relatively new. Radiometric
dating, based on the natural radioactive decay of certain
elements, like carbon, and other techniques revolutionized
the ability to precisely determine the age of ancient rocks in
the second half of the last century.

The developments set the stage for the work of the late
Nobel Prize-winning physicist Luis Alvarez and his geologist
son Walter, professor of Earth and planetary science at the
Univeristy of California, Berkeley. Along with two other
colleagues, they coauthored a sensational 1980
paper about the “iridium anomaly” — a 1-centimeter-thick
(0.4-inch-thick) layer of sedimentary rock rich in iridium, an
element rare on Earth’s surface but common in meteorites.

The researchers attributed the anomaly, which they initially

identified in Italy, Denmark and New Zealand, to the impact
of a large asteroid. They argued the unusual layer
represented the exact moment in time when dinosaurs
disappeared.
First met with skepticism, the iridium anomaly eventually
was spotted in more and more places around the world. A
decade later, a different group of researchers identified the
smoking gun: a 200-kilometer-wide (125-mile-
wide) crater off the coast of Mexico’s Yucatan Peninsula.

The rock and sediment there had a similar composition to

the iridium layers, and the scientists suggested the
depression, called the Chicxulub crater, was caused by the
impact of an asteroid. Researchers believe the other
anomalies spotted across the globe were caused by
scattering debris when the space rock struck Earth.

Most paleontologists now accept that the asteroid caused

what’s known as the end-Cretaceous extinction. The strike
triggered a period of global cooling, with dust, soot and
sulfur thrown up during the impact blocking the sun and
likely shutting down photosynthesis, a key process for life .

One fossil site in North Dakota has provided an

unprecedented level of detail on what that day — and its
immediate aftermath — was like. Debris rained down,
lodging itself into the gills of fish, while huge tsunami-like
surges of water unleashed by the strike killed dinosaurs and
other creatures. Scientists have even figured out that the
asteroid smashed into Earth in springtime .

The disappearance of massive dinosaurs created a world in

which mammals — and ultimately humans — were able to
thrive. And dinosaurs weren’t the total losers they are
sometimes made out to be: Scientists now believe that the
birds that flap around in our backyards directly
evolved from smaller relatives of Tyrannosaurus rex.

In the wake of the Alvarez duo’s stunning discovery, it

initially seemed to scientists as if a space rock impact
might be a general mechanism that explained all
mass extinction events identified in the geological record.
But the end-Cretaceous extinction is the only one reliably
associated with an asteroid, according to Benton.

A different culprit, however, does explain several smaller

extinction episodes and at least two mass extinctions,
including the largest on record.

Apocalyptic volcanoes that caused global warming

Something known as a hyperthermal event — a sudden
warming of the planet — spelled doom for large segments of
life on Earth on more than one occasion. These events have
followed a predictable pattern: volcanic eruption, carbon
dioxide release, global warming, acid rain, ocean
acidification — resulting in a longer road to oblivion than the
dino-killing asteroid but equally destructive.

The biggest mass cataclysm of all time, called the end-

Permian extinction, occurred 252 million years ago. Some
95% of species disappeared on land and at sea as a result of
global warming — with temperatures rising perhaps 10
degrees Celsius to 15 degrees Celsius (18 F to 27 F), Benton
noted in his book.

Known as “the Great Dying ,” the extinction event was

marked by supervolcanic eruptions that expelled greenhouse
gases in an Australia-size region known as the Siberian
Traps in Eurasia. That led to extreme acid rain that killed
plant life and left the land surface rocky as the precipitation
washed rich soil into the oceans, which in turn became
swamped with organic matter, Benton explained.

However, into the void that followed emerged different

creatures that evolved from the survivors, displaying many
new ways of existence with features such as feathers, hair
and speedy locomotion, Benton said.

“One of the big changes … on land, it seems, was a great

rise in energy of everything,” he explained. “All of the
surviving reptiles very rapidly became upright in posture
instead of (low and) sprawling. (Some animals) became
warm blooded in some way because we track feathers back
to the early Triassic dinosaurs and their nearest relatives,
and on the mammals side, we track the origin of hair.”

When dinosaurs got big

Another period of extreme volcanic activity 201 million years
ago marked the end-Triassic mass extinction. It has been
linked to the breakup of the Pangea supercontinent and the
opening of the central Atlantic Ocean. Many land reptiles
vanished as a result of that catastrophic event, making way
for the towering sauropods and armored plant
eaters commonly seen in childhood dinosaur books.

“The dinosaurs were already around but they had not fully
diversified,” Benton said. “And then in the early Jurassic, …
the dinosaurs really took off.”

Deeper in time, a mass extinction event that ended the

Devonian Period, a geological era when life thrived on
land for the first time, was also attributed to a hyperthermal
event likely triggered by volcanic activity 359 million years
ago, according to Benton’s book.

Other research published in 2020 suggested that multiple

star explosions — known as supernovae — may have played
a role.

A less well-understood period of worldwide cooling soon

followed. It’s thought that these twin crises — separated by
only 14 million years — led to rapid changes in temperature
and sea level that resulted in the loss of at least 50% of the
world’s species, wiping out many armored fish, early land
plants, and animals such as the fishapods, or the earliest
elpistostegalians, that were making the transition from
water to land.
The resulting loss of marine species made way for
the golden age of sharks during the Carboniferous Period,
when the predators dominated the seas and evolved to
include a variety of species with different forms.

Sinking temperatures and sea levels

Colder temperatures and a drastic drop in sea levels —
perhaps as much as 10 degrees Celsius (18 F) cooler and
150 meters (492 feet) lower, respectively — played a major
role in the earliest identified mass extinction event, the end-
Ordovician, according to Benton. That shift, which took
place about 444 million years ago, led to the disappearance
of 80% of species at a time when life was mostly limited to
the seas.

What triggered the die-off was the massive Gondwana

supercontinent (today’s South America, Africa, Antarctica
and Australia) drifting over the South Pole during the
Ordovician. When a land mass covers the polar region, the
ice cap reflects sunlight and slows melting, resulting in an
expanding ice cap that lowers sea levels globally.

Adding to the cataclysm was volcanic activity. However, in

this case, it did not appear to make global temperatures
warmer. Instead, phosphorus from lava and volcanic rocks
washed into the sea, gobbling up life-giving oxygen from the
oceans.

The looming sixth mass extinction

A growing number of scientists believe a sixth mass
extinction event of a magnitude equal to the prior five has
been unfolding for the past 10,000 years as humans have
made their mark around the globe.

The dodo, the Tasmanian tiger , the baiji, or Yangtze River

dolphin, and the Western black rhino are just a few of the
species that have disappeared so far in what’s known as the
Holocene or Anthropocene extinction.
While the loss of even one species is devastating, Ceballos
of the National Autonomous University of Mexico has
highlighted that the ongoing episode of extinction is
mutilating much thicker branches of the tree of life, a
metaphor and model that groups living entities and maps
their evolutionary relationships.

Entire categories of related species, or genera, are

disappearing, a process he said is affecting whole
ecosystems and endangering the survival of our own
species.

Ceballos and his study coauthor Paul Ehrlich, Bing Professor

Emeritus of Population Studies at Stanford University,
assessed 5,400 genera of vertebrate animals, excluding
fishes. A single genus groups one or more different but
related species — for example the genus Canis includes
wolves, dogs, coyotes and jackals.

The duo’s analysis found that 73 genera had gone extinct in

the past 500 years. This is much faster than the expected
“background” extinction rate, or the rate at which species
would naturally die off without outside influence — in the
absence of human beings, these 73 genera would have taken
18,000 years to vanish, the researchers said.

The causes of these extinctions are varied — land-use

change, habitat loss, deforestation, intensive farming and
agriculture, invasive species, overhunting and the climate
crisis — but all these devastating changes have a common
thread: humanity.

Ceballos pointed to the extinction of the passenger pigeon,

which was the only species in its genus, as an example of
how losing a genus can have a cascading effect on a wider
ecosystem. The bird’s loss, a result of reckless hunting in
the 19th century, narrowed human diets in eastern North
America and allowed the bacteria-harboring White-footed
mice that were among its prey to thrive.

What’s more, some scientists believe the passenger pigeon’s

extinction, combined with other factors, is behind today’s
rise of tick-borne diseases such as Lyme disease that
plague humans and animals alike, according to the study.

Not only do the destructive actions of humans have the

potential to erode our quality of life in the long term, but
their ripple effects could eventually upend our success as a
species, according to Ceballos.

“When we lose genera, we’re losing more genetic diversity,

we’re losing more evolutionary history, and we’re losing
(many) more ecosystem goods and services that are very
important,” he explained.

While branches of the tree of life are vanishing, the

distribution of certain animal species is becoming more
homogenized — the world is home to about 19.6 billion
chickens, 980 million pigs and 1.4 billion cattle. In some
cases, intensive farming can trigger outbreaks of disease
like avian influenza outbreaks that rip through poultry farms
and increase risk of spillover in wild migratory birds . Other
farm animals act as hosts for virus that infect humans, with
the potential to cause pandemics like Covid-19.

Ultimately, the planet can and will survive just fine without
us, Ceballos added. But, like the iridium anomaly left by the
dinosaur-dooming space rock, what might the final traces of
human civilization look like in the geological record?

Some scientists point to the geochemical traces of nuclear

bomb tests , specifically plutonium — a radioactive element
widely detected across the world in coral reefs, ice cores
and peat bogs.
Others say it could be something altogether more mundane,
such as a fossilized layer of bones from chickens — the
domesticated bird industrially bred and consumed across
the world in mammoth quantities — that’s left as humanity’s
defining legacy for the ages.